Currently, there are a substantial number of various apparatuses and methods for deforming cylindrical thermoplastic pipe liner into a reduced cross-section such that the liner can be inserted into and moved through a pipe to be lined and subsequently expanded into a cylindrical configuration approximating the diameter of the pipe to be lined whereby the thermoplastic material forms the interior surface of the pipe for transporting fluid or other material through the pipe. One such method for protecting the interior of pipe lines is disclosed in French Patent No. 81 07346 to Laurent. In the patent, there is disclosed a cylindrical liner formed of a thermoplastic material having historic time-memory properties, i.e., which enables in the reforming process adaptation to given shape-memory characteristics. The term "shape-memory" shall mean that the liner is reformed to fit given configurations of the internal shape, typically the cross-sectional shape, of an object, such as circular, oval, rectilinear and the like and that the thermoplastic property of "historic time memory" is the means by which this may be accomplished. That is, the liner is initially formed in a cylindrical configuration and is deformed at an appropriate temperature to reduce the cross-section of the liner, enabling introduction of the deformed liner into a pipe to be lined. The deformed liner in the pipe line is then expanded by the introduction of pressurized hot fluid, e.g., steam, and at a temperature such that the deformed liner assumes its original cylindrical shape or the shape of the host pipe. With the applied steam pressure, the liner conforms to the interior contour of the pipe. Various cross-sectional shapes of the deformed liner are disclosed in the Laurent patent, namely, "U," "H," "V," and star-shaped configurations.
Liners having generally U-shaped cross-sectional configurations, as disclosed in this French patent, have been successfully used commercially by the assignee of the present application (also the assignee of the Laurent patent) and various innovations have been made by assignee with respect to the application of U-shaped pipe liners of the French patent in a commercial sense, for example, see U.S. Pat. Nos. 3,863,365; 4,985,196; 4,986,951; 4,998,871; 5,091,137 and 5,112,211. The focus of those U.S. patents, as well as that of the Laurent patent, has been on the applicability of the shape-memory characteristics of polyethylene liner material to a generally U-shaped deformed liner for subsequent reformation or expansion within a generally cylindrical pipe to provide a lined pipe.
Other types of liners are also known. For example, in British Patent Specification 1580438, as well as in PCT application PCT/AU86/00362, WO87/03840, there are disclosed liners having generally U-shaped configurations (an X-configuration being further shown in British Patent No. 1,580,438). The liners of these documents employ semi-rigid plasticized material in cylindrical form which is forcibly mechanically collapsed, i.e., folded to form one or more depressions extending longitudinally along the length of the pipe, to provide a generally U-shaped or X-shaped configuration in cross-section. The mechanically deformed liner is maintained in the reduced cross-sectional shape by strapping. Later, when the reduced cross-section liner is installed in a pipe, the strapping is released such that the deformed liner returns to its original cylindrical configuration, i.e., spring rebounds back to its original configuration, to line the interior of the cylindrical pipe.
In U.S. Pat. No. 5,034,180, there is disclosed a method for installing a liner of plastic material in a pipe. In that patent, a thin flexible tubular membrane providing a heat containment tube is inserted into an underground pipe to be repaired. The thermally deformed liner is inserted into the tube while hot and flexible. The tubular membrane is pressurized to exclude unwanted fluids from within the existing pipe. Hot steam is injected into the tube to heat the pipe for its full length externally and internally until it assumes its original circular shape, the liner thereby being expanded against the walls of the pipe with a flexible tubular membrane therebetween.
All of these known methods deform or mechanically collapse the generally cylindrical plastic liners into a reduced cross-sectional shape enabling the deformed liner for insertion into the pipe being lined. The thermoplastic liner uses the shape-memory characteristics both in manufacturing the liner, as well as later during the reformation process to reconstitute the liner to its cylindrical configuration. The mechanically deformed liners rely on a manufacturing process which introduces substantial stress into the liner and which inhibits the formation of liners for lining large-diameter pipes and restricts the wall thickness of the liner. These two processes are both restricted to liners which do not exceed a pre-given wall thickness and this also affects the reliability of the lined pipe in certain instances. Other processes are heat/pressure processes which soften the material to the point of melting and, as such, are non-repeatable thermoplastic processes.
Several significant problems also exist with respect to the formation, coiling and reformation of the previously utilized liners such as the U-liner.TM. installations. For example, stresses introduced into the liner during deformation and coiling are oftentimes difficult to relieve upon installation. Flat spots sometimes occur in the reformed pipe, i.e., a wall portion of the reformed, generally cylindrical liner remains flat and spaced from the pipe wall being lined due to small circumferential length of the originally bent portion. For example, in the U-shaped liner, sharp bends over a small circumferential length as at the concave bent apex or peak of the U-shaped liner produce very high residual stresses. That is, in a U-shaped liner, its concave peak has the highest magnitude of residual stresses and the flat spot is sometimes formed from inadequate relief of the stresses at that concave peak. Also, the ability to coil long lengths of the deformed liner is very significant from a practical and commercial point of view. Large-diameter liners, of course, without substantial reduction in their deformed shape cannot be coiled in substantial lengths. This constitutes a significant limitation on the use of liners of this type for lining large-diameter pipes. That is to say, notwithstanding the reduction in cross-section, for example, afforded by a U-shaped liner, for large-diameter liners there remains the problem of an ability to coil and carry appropriate lengths of the deformed liner for later transport to the installation site. The reduction in cross-sectional size of the U-shaped liner, while significant, and approximating a 35% reduction in cross-section from the extruded cylindrical shape of the liner, is still not a sufficient reduction to enable long lengths of large-diameter liner, e.g., 18-50 inch diameter liners, to be coiled. Even with this significant reduction in cross-section of a U-shaped liner, there remains problems associated with pulling the liner through the pipe, particularly at bends in the pipe or areas of reduced cross-sections and local imperfections.